This invention relates to electronic timepieces. More particularly, the invention relates to a circuit for use in adjusting the rate of an electronic timepiece for changes due to temperature, changes and the like.
A number of methods are known by means of which the rate of a timepiece which uses a quartz crystal as a time base can be adjusted to compensate, for example, for changes in temperature. In one such arrangement, a trimmer capacitor is added to the oscillator circuit and the frequency of oscillation is varied by increasing or decreasing the capacitive reactance of the circuit. In another widely used arrangement, known as the digital tuning method, a variable divider is provided at the output of the oscillator for changing the driving rate of the timepiece.
In recent years, however, even though timepieces have become more and more precise, there remains a strong demand for even more precision, accompanied by high reliability. The achievement of this result by means of the above-described arrangements is extremely difficult.
One solution to this problem has been put forth which uses the basic circuit shown in FIG. 2 in which a frequency changing capacitor 2 is connected, via transistor switch 3, to a quartz crystal oscillator 1 which provides a standard frequency for the timepiece. The frequency of oscillation is changed by opening and closing switch 3, thereby adjusting the rate of the timepiece. Details of this method are disclosed, for example, in U.S. Pat. Nos. 3,568,093 and 4,473,303. This method is advantageous in that precise rate adjustment can be done quickly. When combined with the aforementioned digital tuning method, a rate adjustment is achieved which is effective over a wide temperature range.
However, the above method has defects of its own arising out of the fact that the capacitance of the switched capacitor is small and the capacitor is usually incorporated with the transistor switch in an integrated circuit. This results in the variation, from timepiece to timepiece, in the capacitance value realized at the time of manufacture, resulting in undesirable variations in the change in frequency of oscillation produced when the capacitor is connected to the oscillator. Thus, when the operation of the transitor switch is to be adjusted by equal adjustments of rate at the initial timing of the movement, the effect of the adjustment varies, as between individual timepieces, thereby making the rate adjustment troublesome.
The foregoing is especially true when, after distribution of the timepieces by the manufacturer, additional rate adjustment of the timepieces is required in order to compensate for the effects of aging and the like. This adjustment is difficult for the average retailer to perform since, due to the difficulty of attaching data to each individual timepiece, the necessary data for adjusting the rate has not accompanied the timepiece. Because the retailer does not know to exactly what extent the rate of the timepiece can be adjusted by the method he is accustomed to using, he must use a rate measuring device and the focusing method to bring the timepiece gradually nearer to the desired rate. The result is that, due to the generally very troublesome rate adjustment, a large amount of costly retailer time must be spent in a non-retailing activity.
Variation of the rate of the quartz crystal oscillator of FIG. 2, having a second order frequency-temperature characteristic, is illustrated in FIGS. 3(a) and 3(b). FIG. 3(a) shows the effect on the frequency-temperature characteristics of a timepiece when the rate-changing capacitor has too small a capacitance, while FIG. 3(b) shows the effect of too large capacitance, as the result of variations in manufacture, the quartz crystal oscillator otherwise having identical characteristics. The frequency-temperature curve of each timepiece, when switch 3 is open, is marked f.sub.0 (t). When switch 3 is closed, the curves of frequency vs. temperature are respectively marked f.sub.1 (t) and f.sub.2 (t). Frequency changes .DELTA.f.sub.1 and .DELTA.f.sub.2 are produced by opening and closing the respective transistor switch. The amount of frequency change .DELTA.f.sub.1 or .DELTA.f.sub.2 is generally constant over the whole desired temperature range. If it is assumed that the rate adjustment is to be done in four adjusting steps, division of the distance .DELTA.f.sub.1 (between peaks of the second order curves f.sub.1 (t) and f.sub.0 (t)) into four steps, results in different step widths for the timepiece of FIG. 3(a) as compared to that of FIG. 3(b). Thus, the manufacturing variation in the capacitance of the added capacitor causes the width of one adjusting step, as between individual timepieces, to differ and makes it impossible to predict how many steps will be required for a rate adjustment. At present, therefore, the rate must be adjusted by the focusing method, as previously stated.